Generally described, computing devices utilize a communication network, or a series of communication networks, to exchange data. Companies and organizations operate computer networks that interconnect a number of computing devices to support operations or provide services to third parties. In some instances, computing networks may be used to transmit items of digital content to users for consumption on a user's computing device. For example, a user's computing devices may visually convey items of content such as animations, electronic books, electronic periodicals, movies, television programs, multi-media content and portions thereof on an electronic screen or touchscreen. A user's computing devices may also direct audible output through headphones or speakers to convey items of audible content such as audiobooks, songs, movies, television programs, multi-media content and portions thereof.
Digital content may be utilized by users in a variety of contexts. For example, users desiring to learn a language may utilize digital content in that language to increase the user's exposure to the language, with the goal of increasing the user's fluency. In language learning applications, the difficulty of a content item (e.g., due to the specific vocabulary used or the nature of presentation of the language) can greatly affect the ability of a user to comprehend the item. However, the perceived difficulty of an item may vary between individual users. Consequently, selecting digital content of an appropriate difficulty can be challenging to users.
In addition, some content items are available to users in a variety of formats, such as in both textual and audio formats. However, a user's fluency level within a language may vary according to the format in which content is presented. For example, a native language learner may have a higher fluency with respect to spoken language than with respect to written text. Conversely, a non-native language learner may have a higher fluency with respect to written text than with respect to spoken language.